#14983: always add a line end after a MIME boundary marker.

This is more RFC compliant (see issue) and fixes a problem with
signature verifiers rejecting the part when signed.  There is some
amount of backward compatibility concern here since it changes
the output, but the RFC issue coupled with fixing the problem
with signature verifiers seems worth the small risk of breaking
code that depends on the current incorrect output.

Lib/email/generator.py

@@ -299,9 +299,8 @@ class Generator:
             # body-part
             self._fp.write(body_part)
         # close-delimiter transport-padding
-        self.write(self._NL + '--' + boundary + '--')
+        self.write(self._NL + '--' + boundary + '--' + self._NL)
         if msg.epilogue is not None:
-            self.write(self._NL)
             if self._mangle_from_:
                 epilogue = fcre.sub('>From ', msg.epilogue)
             else:

Lib/test/test_email/data/msg_02.txt

@@ -119,6 +119,7 @@ hello
 
 
 --__--__----
+
 --192.168.1.2.889.32614.987812255.500.21814
 Content-type: text/plain; charset=us-ascii
 Content-description: Digest Footer

Lib/test/test_email/test_email.py

@@ -1711,7 +1711,8 @@ From: bperson@dom.ain
 
 --BOUNDARY
 
---BOUNDARY--''')
+--BOUNDARY--
+''')
 
     def test_no_parts_in_a_multipart_with_empty_epilogue(self):
         outer = MIMEBase('multipart', 'mixed')
@@ -1756,7 +1757,8 @@ MIME-Version: 1.0
 Content-Transfer-Encoding: 7bit
 
 hello world
---BOUNDARY--''')
+--BOUNDARY--
+''')
 
     def test_seq_parts_in_a_multipart_with_empty_preamble(self):
         eq = self.ndiffAssertEqual
@@ -1782,7 +1784,8 @@ MIME-Version: 1.0
 Content-Transfer-Encoding: 7bit
 
 hello world
---BOUNDARY--''')
+--BOUNDARY--
+''')
 
 
     def test_seq_parts_in_a_multipart_with_none_preamble(self):
@@ -1808,7 +1811,8 @@ MIME-Version: 1.0
 Content-Transfer-Encoding: 7bit
 
 hello world
---BOUNDARY--''')
+--BOUNDARY--
+''')
 
 
     def test_seq_parts_in_a_multipart_with_none_epilogue(self):
@@ -1834,7 +1838,8 @@ MIME-Version: 1.0
 Content-Transfer-Encoding: 7bit
 
 hello world
---BOUNDARY--''')
+--BOUNDARY--
+''')
 
 
     def test_seq_parts_in_a_multipart_with_empty_epilogue(self):

Misc/NEWS

@@ -48,6 +48,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #14983: email.generator now always adds a line end after each MIME
+  boundary marker, instead of doing so only when there is an epilogue.  This
+  fixes an RFC compliance bug and solves an issue with signed MIME parts.
+
 - Issue #20540: Fix a performance regression (vs. Python 3.2) when layering
   a multiprocessing Connection over a TCP socket.  For small payloads, Nagle's
   algorithm would introduce idle delays before the entire transmission of a